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It is always a bit of a gamble heading into flu season every year since flu vaccines – whose strains must be selected 6 months in advance – may not end up being a very good match for the viruses that are circulating in the fall. Flu viruses mutate over time – and minor strains that seemed insignificant last spring can become dominant by November.
Such is the the case this year, as a recently arrived, `drifted’ H3N2 flu virus (see CDC HAN Advisory On `Drifted’ H3N2 Seasonal Flu Virus) has come on like gang busters over the past few months, beating out the `vaccine’ strain by a factor of 2 to 1.
The result, as you might expect, is that this year’s flu vaccine isn’t as effective as we’d hoped.
As we’ve discussed before, flu vaccines – while considered very safe – most years only offer a moderate level of protection against influenza. Their VE (vaccine effectiveness) can vary widely between flu shot recipients, and is often substantially reduced among those older than 65 or those with immune problems.
As an example, in October of 2011, in CIDRAP: A Comprehensive Flu Vaccine Effectiveness Meta-Analysis, we saw a major review indicating the TIV (Trivalent Influenza Vaccine) - during 8 of 12 flu seasons (67%) – produced a combined efficacy of only 59% among healthy adults (aged 18–65 years).
Despite these moderate success levels, studies have shown the benefits of flu vaccination – even if those benefits aren’t as universal, or predictable, as we’d like.
- Last May, in CDC: Flu Shots Reduce Hospitalizations In The Elderly, we saw a study that suggested that even when a vaccine’s effectiveness is low, it can help attenuate the severity of influenza in the elderly.
- Similarly, over the past couple of years, we’ve seen studies suggesting the flu vaccine may reduce the risk of heart attack and stroke (see JAMA: Flu Vaccine and Cardiovascular Outcomes & Study: Flu Vaccine May Reduce Heart Attack).
So, given its limitations, you may be wondering why I bother to get the flu vaccine every year and recommend it to others.
I consider it cheap insurance, even if the shot only offers a moderate degree of protection. Just as wearing a seatbelt doesn’t guarantee you’ll walk away from a wreck, a flu vaccine won’t guarantee you’ll stay flu-free for the season. But it can improve your odds.
Some years, obviously, more than others.
All of which leads up to today’s MMWR report, which carries a mid-season estimate of the effectiveness of this year’s flu vaccine – one that comes in at an admittedly disappointing 23%.
First the CDC’s press release, followed by a link to the MMWR report.
Protection from flu vaccination reduced this season
CDC urges early treatment of severely ill and high-risk patients
A report published in the January 16 Morbidity and Mortality Weekly Report (MMWR) estimates that getting a flu vaccine this season reduced a person’s risk of having to go to the doctor because of flu by 23 percent among people of all ages.
Since CDC began conducting annual flu vaccine effectiveness (VE) studies in 2004-2005, overall estimates for each season have ranged from 10 percent to 60 percent effectiveness in preventing medical visits associated with seasonal influenza illness. The MMWR report says this season’s vaccine offers reduced protection and this underscores the need for additional prevention and treatment efforts this season, including the appropriate use of influenza antiviral medications for treatment.
“Physicians should be aware that all hospitalized patients and all outpatients at high risk for serious complications should be treated as soon as possible with one of three available influenza antiviral medications if influenza is suspected, regardless of a patient’s vaccination status and without waiting for confirmatory testing,” says Joe Bresee, branch chief in CDC’s Influenza Division. “Health care providers should advise patients at high risk to call promptly if they get symptoms of influenza.”
One factor that determines how well a flu vaccine works is the similarity between the flu viruses used in vaccine production and the flu viruses actually circulating. During seasons when vaccine viruses and circulating influenza viruses are well matched, VE between 50 and 60 percent has been observed. H3N2 viruses have been predominant so far this season, but about 70 percent of them have been different or have “drifted” from the H3N2 vaccine virus. This likely accounts for the reduced VE.
Flu viruses change constantly and the drifted H3N2 viruses did not appear until after the vaccine composition for the Northern Hemisphere had been chosen.
Another factor that influences how well the flu vaccine works is the age and health of the person being vaccinated. In general, the flu vaccine works best in young, healthy people and is less effective in people 65 and older. This pattern is reflected in the current season early estimates. VE was highest -- 26 percent -- for children age 6 months through 17 years. While not statistically significant, VE estimates for other age groups were 12 percent for ages 18 to 49 years and 14 percent for people age 50 years and older.
CDC recommends that people get a flu vaccine even during season’s when drifted viruses are circulating because vaccination can still prevent some infections and can reduce severe disease that can lead to hospitalization and death. Also, the flu vaccine is designed to protect against three or four influenza viruses and some of these other viruses may circulate later in the season. Flu activity so far this season has been similar to the 2012-2013 flu season, a “moderately severe” flu season with H3N2 viruses predominating.
Antiviral Supply Update
While manufacturers of antiviral medications have stated that there is no national shortage of antiviral medications at this time, and that there is sufficient product available to meet high demand, there are anecdotal reports of spot shortages of these drugs. CDC’s advice for patients and doctors is that it may be necessary to contact more than one pharmacy to fill a prescription for an antiviral medication. Pharmacies that are having difficulty getting orders filled should contact their distributor or the manufacturer directly.
For large institutional outbreaks this season, CDC is taking new measures to help match demand with supply, working with commercial partners to facilitate filling of large orders of antivirals for long-term care facilities or institutions having difficulty accessing antiviral supplies in outbreak settings. More information is available at http://www.cdc.gov/flu/antivirals/supply
Early Estimates of Seasonal Influenza Vaccine Effectiveness — United States, January 2015
Weekly
January 16, 2015 / 64(01);10-15Brendan Flannery, PhD1, Jessie Clippard, MPH1, Richard K. Zimmerman, MD2, Mary Patricia Nowalk, PhD2, Michael L. Jackson, PhD3, Lisa A. Jackson, MD3, Arnold S. Monto, MD4, Joshua G. Petrie, MPH4, Huong Q. McLean, PhD5, Edward A. Belongia, MD5, Manjusha Gaglani, MBBS6, LaShondra Berman, MS1, Angie Foust, MA1, Wendy Sessions, MPH1, Swathi N. Thaker, PhD1, Sarah Spencer, PhD1, Alicia M. Fry, MD1 (Author affiliations at end of text)
In the United States, annual vaccination against seasonal influenza is recommended for all persons aged ≥6 months (1). Each season since 2004–05, CDC has estimated the effectiveness of seasonal influenza vaccine in preventing medically attended acute respiratory illness (ARI) associated with laboratory-confirmed influenza. This season, early estimates of influenza vaccine effectiveness are possible because of widespread, early circulation of influenza viruses.
By January 3, 2015, 46 states were experiencing widespread flu activity, with predominance of influenza A (H3N2) viruses (2). This report presents an initial estimate of seasonal influenza vaccine effectiveness at preventing laboratory-confirmed influenza virus infection associated with medically attended ARI based on data from 2,321 children and adults enrolled in the U.S. Influenza Vaccine Effectiveness Network (Flu VE) during November 10, 2014–January 2, 2015. During this period, overall vaccine effectiveness (VE) (adjusted for study site, age, sex, race/ethnicity, self-rated health, and days from illness onset to enrollment) against laboratory-confirmed influenza associated with medically attended ARI was 23% (95% confidence interval [CI] = 8%–36%). Most influenza infections were due to A (H3N2) viruses.
This interim VE estimate is relatively low compared with previous seasons when circulating viruses and vaccine viruses were well-matched and likely reflects the fact that more than two-thirds of circulating A (H3N2) viruses are antigenically and genetically different (drifted) from the A (H3N2) vaccine component of 2014–15 Northern Hemisphere seasonal influenza vaccines (2). These early, low VE estimates underscore the need for ongoing influenza prevention and treatment measures. CDC continues to recommend influenza vaccination because the vaccine can still prevent some infections with the currently circulating A (H3N2) viruses as well as other viruses that might circulate later in the season, including influenza B viruses. Even when VE is reduced, vaccination still prevents some illness and serious influenza-related complications, including thousands of hospitalizations and deaths (3). Persons aged ≥6 months who have not yet been vaccinated this season should be vaccinated, including persons who might already have been ill with influenza this season.
